1. Field of the Invention
The present invention relates to a portable information terminal apparatus, and more particularly a portable information terminal apparatus which can correctly detect a power supply voltage.
2. Description of the Related Art
A conventional example of a detecting system for a power supply voltage in an apparatus which uses a battery as a power supply is described in, for example, Japanese Laid Open Patent Disclosure (JP-A-Heisei 4-171429: hereinafter, to be referred to as a first conventional example). In the first conventional example, a current is flows through a load from the power supply during the detection of the power supply voltage and then the power supply voltage is determined based on the resistance of the load and the current value at that time.
FIG. 1 is a block diagram illustrating the structure of the detecting system in the above first conventional technique. Referring to FIG. 1, a CPU 102 first reads a duty factor, either a heavy load duty (for determining a maximum remaining power quantity) and a light load duty (for determining whether the remaining power quantity is sufficient) from an EEPROM 103. Next, the CPU 102 drives a transistor 104 with the heavy load duty which has been read from the EEPROM 103. It should be noted that a capacitor 107 and a diode 108 form a power supply filter. Because a dummy load current flows through a dummy load 105, the battery voltage (Vcc) reduces gradually. At this time, the voltage (Vdd) which is applied to a voltage detection IC 106 also reduces. In this case, a CPU 102 determines whether or not the output of the voltage detection IC 106 reaches a determination voltage. When the output of the voltage detection IC 106 is left in the high level, it is determined whether or not it passed for 10 ms after the supply of current to the dummy load 105 is started. When not passing for 10 ms, the control returns to the determination of whether or not the output of the voltage detection IC 106 reaches the determination voltage. In a case where the above-mentioned ON detecting means is used, the decrease of battery voltage (Vcc) is little when the remaining power of a battery 101 is maximum. Therefore, the voltage which is applied to the voltage detection IC 106 does not also reach the determination voltage. In accordance with, after the time period of 10 ms elapsed, the CPU 102 determines that the remaining power quantity is large and the result is displayed to the user. After that, the supply of current to the dummy load 105 is stopped and the detection of the battery voltage is ended.
However, in the first conventional example of the detecting system mentioned above, an unnecessary current is flowed to the dummy load in the detection of the battery voltage and the timing when the battery voltage is to be detected is not considered. Therefore, when the battery power capacity is sufficient, there is a case that the voltage detection is frequently performed. For this reason, there is a problem in that the lifetime of the battery has become short because the battery power is unnecessarily consumed for the detection of the battery voltage.
Further, there is the possibility that the battery voltage decreases by connecting the dummy load so that the circuit operates erroneously when the remaining power quantity is not enough.
The reason is that there is a case that the battery can supply current enough to operate a low power consumption circuit even if the remaining power quantity of the battery is decreased. At that time, the battery voltage for a peripheral circuit temporarily decreases, because a large amount of current flows temporarily when the power starts to be supplied from the battery to the dummy load.
Another reason is that because the power supply and/or a clock signal are turned ON or OFF, the load of the power supply changes so that the detection of the battery voltage can not be correctly performed.
These reasons will be specifically described with reference to FIG. 1 again. In the first conventional example shown in FIG. 1, because the load 109, which represents the operation state of a peripheral circuit, is not constant, the value of the current which flows through the load 109 changes and the detection of the battery voltage can not be correctly performed.